Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material including metal halide additives and a rare gas to facilitate starting. Mercury may also be included. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
To produce a practical lamp it is of course necessary to mount the arc tube within its outer envelope and the mounts formerly employed have a greater number of components and, therefore, more welds. Efforts to achieve automated mount assembly have not been reasonably successful and the failure of such efforts has added to the cost of the lamps. Also, in the past, lamps of different wattage had to have a specific mount structure, which also added to the cost.
It is also important that the mount does not cause or contribute to sodium migration from the arc tube, a frequent occurrence that deleteriously affects the light output of the lamp.